The present invention relates to an improved apparatus and method for forming structural preforms.
From the sports industry to the marine and the automotive industry, composite materials are being pursued as the material of choice for their low weight and cost effective production methods. Reinforcing fibers can be combined to make many different fiber architectures. Based on structural integrity and fiber linearity and continuity, fiber architectures can be classified into four categories: discrete, continuous, planar interlaced (2-D), and fully integrated (3-D) structures.
A discrete fiber system like a fiber mat has the least reinforcement material continuity. The continuous filament or unidirectional (0xc2x0) system has the highest level of fiber continuity and linearity, and consequently has the highest level of property translation efficiency from the fiber to the composite product. The drawback of this fiber architecture is its intra-and inter-laminar weakness due to the lack of in-plane and out-of-plane yarn inter-lacings. The planar interlaced or inter-looped systems, which include architectures like 2-D woven and knitted fabrics, address the intra-laminar failure problem but the inter-laminar strength is still limited by the matrix strength due to the lack of through thickness fiber reinforcements. The last category includes fully integrated systems that have a tree dimensional network of yarn bundles.
Textile preforming provides a link between raw material systems and the molding of the composite product. Depending upon the textile preforming method used, the range of fiber orientation and fiber volume fraction of the preform will vary, subsequently affecting matrix infiltration and consolidation and therefore can change the structural performance of the composite product.
Currently, low-cost preforming in the automotive industry is generally done using either a variation of the directed fiber process or by matched die forming of a thermoformable continuous strand mat, with little effort concentrating on structural fabrics. The existing preforming techniques meet high rate and low cost requirements, however, the structural requirements for a highly loaded, low weight composite part cannot be reached easily without the use of continuous fiber reinforcement fabrics. Following is a list of current preforming techniques:
Discontinuous Fiber Preforming
Directed chopped fiber with polyester binder;
Programmable powder preform process xe2x80x9c(P4)xe2x80x9d, automated chopper gun;
Dow thermal spray chop gun (manual and robotic);
Chopped matxe2x80x94Matched tool thermoforming;
Chopped fiber slurry
Continuous Fiber Preforming
Continuous Strand Mat (CSM)xe2x80x94Matched tool thermoforming
Stitching
Hand Lay-up
Dry filament winding
Braiding
CompForm (Mat, multi warp knit xe2x80x9c(MWK)xe2x80x9d, Woven)
Diaphragm thermoforming (Mat, MWK, Woven)
Most of the current techniques for forming engineered fabrics make use of the mold tool, hand lay-up of plies, vacuum bag consolidation and convection oven heating methods that prolong the forming cycle. Rarely is a tool string dedicated specifically to forming the reinforcing fabric, with the exception of the CompForm process which requires matched die sets. Stitching methods attach one layer of fabric to the next or connect one piece to another, but in either instance, the resultant preform is limp and unconsolidated. Random continuous and discontinuous glass mat is often formed at high rate using matched die thermoforming. The random fiber orientation limits application of the resulting composite to secondary structural applications where fiber loading of 45 percent by weight is sufficient.
Other techniques for preform fabrication are based on discontinuous or chopped fibers. These techniques use a screen and require fluid (air or water) flow to trap the fiber against the porous tool surface. Air born, directed fiber systems are improving with respect to controlling fiber angle and placement, culminating in the xe2x80x9cP4xe2x80x9d process. Manual chopper gun/screen methods have been used since the 1950s. By maintaining the fiber alignment, stiffness remains in tack, but in all cases the discontinuity and misalignment of the fiber lowers the maximum theoretical fiber volume and greatly reduces the laminate strength.
In a first embodiment, Applicants"" novel apparatus comprises a lay-up station and a forming station. A shuttle table moves bidirectionally from the lay-up station into the forming station and back to the lay-up station. A forming tool comprising a first mold part which defines the 3-dimensional shape of the desired structural preform is disposed on the shuttle table.
The forming station includes two independently movable horizontal platens. A press platen includes an elastomeric diaphragm which functions as a second mold part. An emitter platen disposed above the press platen includes a plurality of electromagnetic energy emitting devices.
An electromagnetic energy curable binder composition is applied to one or more layers of reinforcing material and that coated reinforcing material is laid up onto the forming tool in the lay-up station. The shuttle table is then moved into the forming station and secured in place. The press platen is lowered such that the elastomeric diaphragm stretches to encapsulate the coated reinforcing material/forming tool thereby forming the coated reinforcing material into the shape of the preform.
The spatial orientation of the plurality of energy emitting devices is adjusted and the emitter platen is then lowered to near vicinity of the stretched elastomeric diaphragm. Electromagnetic energy is directed through the elastomeric diaphragm into the binder coated reinforcing material to heat and form the structural perform under pressure. After the curing cycle is complete, the diaphragm is cooled. In another embodiment, the elastomeric diaphragm is also cooled during the curing cycle.
After completion of the curing and cooling cycles, the shuttle table is returned to the lay-up station and the structural preform is removed from the forming tool. As those skilled in the art will appreciate, movement and securing of the shuttle table may be performed manually or may be automated.
In another embodiment, a second lay-up station is added to the apparatus described above. In this embodiment, one lay up station can be used to lay-up the uncured reinforcing material onto the forming tool where the material is heated and formed under pressure into a structural perform, and the other lay-up station used to remove the formed structural preform from the forming tool.
In another embodiment, Applicants"" apparatus comprises a plurality of lay-up stations alternating with a plurality of forming stations as above described. In this embodiment, more complex preforms can be manufactured by sequential lay-up and forming operations.